Solid-state image-pickup device with column-line amplifiers and limiter

ABSTRACT

The levels of the power supply and the ground are kept constant against a parasitic resistance by keeping the constant current of an amplifier irrespective of the size of a pixel signal and the gain of the amplifier in this case in an image signal reading circuit system having the amplifier arranged to each column. The amplifier has a limiter at an output terminal thereof which limits an output voltage of the amplifier to a range for keeping the constant consumption-current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pick-up device anda pixel signal reading method of the solid-state image pick-up device.

2. Description of the Related Art

Recently, a CMOS image sensor is focused as an image sensor in place ofa CCD. Because the CMOS image sensor solves such problems of the CCDthat the manufacturing thereof needs a dedicated process, the operationneeds a plurality of power voltages, and the operation requires thecombination of a plurality of peripheral ICs to complicate the system.

The CMOS image sensor uses the same manufacturing process as that of ageneral worldwide produced CMOS integrated circuit (IC), and is drivenby a single power-supply. Further, the CMOS image sensor mixedlyincludes an analog circuit and a logical circuit using the CMOS processin the same chip and therefore has a plurality of effective merits toreduce the number of peripheral ICs.

Mainly, an output circuit of the CCD uses a one-channel output with anFD (Floating Diffusion) amplifier. On the contrary, the CMOS imagesensor has the FD amplifier for each pixel and mainly selects one row ina pixel array, as an output, and simultaneously reads the pixels in thecolumn direction, that is, outputs the data in parallel with the column.Because the FD amplifier arranged in the pixel does not sufficientdriving capacity, then, the reduction in data rate is necessary, and theparallel processing is advantageous.

Further, serving as another merit, since the parallel processingsuppresses the band of a signal output circuit, and the noise level isthus reduced.

Various signal output circuits of the parallel-output-type CMOS imagesensor are proposed. For example, the pixel output is sampled by aswitching capacitor and then is read, or the pixel output is read withthe amplifier arranged to each column, depending on cases, with an ADconverter or a DRAM arranged to each column. The present inventionparticularly relates to a pixel signal reading method with an amplifierarranged to each column.

Japanese Unexamined Patent Application Publication No. 5-207220discloses an example of the pixel signal reading method having theamplifier (e.g., single-end amplifier) arranged to each column. Thiswill be described with reference to FIGS. 9 and 10.

FIG. 9 shows only one column corresponding to one pixel GS (circuitsystem of one vertical signal line VL).

The pixel GS comprises: a photodiode PD; a reset transistor Trst; anamplifying transistor Tg; and a reading transistor Ts.

In this case, an output from the pixel GS is read by using a chargeintegrating circuit having the capacitors C1 and C2 and a source groundamplifier 100 serving as a charge integrating amplifier. The sourceground amplifier 100 comprises: a driving MOS transistor Tr16 and a loadMOS transistor Tr17.

Further, in this case, the capacitor C2 serving as a feedbackcapacitance is pre-charged by a transistor Tr15 for switching and areference voltage Vref. Thus, the variation in offset of the sourceground amplifier 100 is suppressed.

FIG. 10 shows a timing chart of the circuit shown in FIG. 9. For a termT1 of a horizontal blanking period, the pixel GS outputs, to a verticalsignal line VL, a value which is obtained by overlapping a signal Vps toan offset voltage Vo. The charge integrating circuit is reset byswitching on a transistor Tr13 by a signal ARC. Similarly, the signalφRC switches on the transistor Tr15 and a signal φTC switches off atransistor Tr14, and thus the capacitor C2 pre-charges the referencevoltage Vref.

For a term T2, the signals φRC and φTC switch off the transistor Tr15and switches on the transistor Tr14, and thus the pre-charged referencevoltage Vref appears at the output Vout of the charge integratingamplifier. In this case, the transistor Tr13 is switched offsimultaneously with the transistor Tr15 and thus the reset state iscanceled.

For a term T3, only the offset voltage Vo is outputted from the pixel GSand is integrated, thereby reading only a signal component of the outputof the charge integrating amplifier using a ratio of the capacitors C1and C2, serving as a gain.

Finally, the read signal is sequentially outputted to a horizontalsignal line HL synchronously with a pulse SR supplied from a shiftregister 101.

In the operation for reading the pixel signal with the amplifierarranged to each column, the offset voltage of the pixel is removed andonly the signal component is easily extracted. Further, the reading gainis set by the ratio of the capacitors C1 and C2. In addition,advantageously, the variation in source followers is suppressed bypre-charging the reference voltage Vref.

Another example will be described with reference to FIGS. 11, 12, and13. This example is jointly announced as “A Column-BasedPixel-Gain-Adaptive CMOS Image Sensor for Low-Light-Level Imaging” byShizuoka University and Sony Corporation in ISSCC (InternationalSolid-State Circuits Conference) in 2003.

Similarly to FIG. 9, FIG. 11 shows only one column corresponding to onepixel GS (circuit system of one vertical signal line VL).

In the example shown in FIG. 11, similarly to FIG. 9, the signal fromthe pixel GS is read by a single-end amplifier AP and a chargeintegrating amplifier comprising capacitors C1 and C2.

The capacitor C2 comprises capacitors c21 and c22 and a switch Sφ3, andvaries the capacitance depending on the switching-on/off operation ofthe switch Sφ3. Thus, the reading gain of the amplifier AP varies.

FIG. 12 shows a circuit example of the amplifier AP. Basically, theamplifier AP is a source ground amplifier comprising an N-channel MOSdriving transistor T11 and a P-channel MOS current-source loadtransistor T10. Further, the amplifier AP has the structure of regulatedcascade, that is, having cascade-connected transistors T12 and T13 andauxiliary amplifiers As1 and As2. With the structure, the excessivelyhigher gain is obtained as compared with the source ground amplifier 100shown in FIG. 9.

FIG. 13 shows a driving timing.

Pulses φV, φR, and φTX drive the pixel GS. Pulses φ1 to φ4 switch-on/offswitches Sφ1 to Sφ4 shown in FIG. 11.

In this case, the same charge integrating amplifier is used andtherefore the basic operation is similar to that shown in FIG. 9. Thepixel GS shown in FIG. 11 is an example of a pixel with a transfer gateTt and, in this case, as will be apparently understood by the drivingtiming shown in FIG. 13, the reset operation is performed by the pulseφR, the offset voltage Vo as the reset level is read, and then a valueobtained by overlapping the signal level Vps to the offset value Vo bythe pulse φTX is outputted.

This order in the example is reverse to that shown in FIG. 9. However,in the example shown in FIG. 11, a KT/C noise at the reset timing due tothe pulse φR is removed and therefore the noises are reduced. Theexample shown in FIG. 11 recently becomes a mainstream. Incidentally,the polarity of the signal output in the example shown in FIG. 11 isinverted to that shown in FIG. 9. Further, the example shown in FIG. 11does not include the operation for pre-recharging the reference voltageVref, the pulse φ1 switches-on the switch Sφ1, and a signal is outputtedby using a threshold voltage for feedback as the reference. The outputvoltage Vout is [Vt+(C1/C2)·Vps].

For example, the examples shown in FIGS. 9 and 11 are known. In theoperation for reading the pixel signal with the amplifier arranged toeach column as mentioned above, advantageously, the offset voltage ofthe pixel is removed and only the signal component is extracted withoutany losses. Further, the reading gain is arbitrarily set by the ratio ofthe capacitances.

However, the amplifier is laid-out to each column and therefore thelayout area increases. The circuit structure per column must besimplified as much as possible. As described above, preferably, therelatively simple source-ground amplifier is used.

Although a differential amplifier can be used, preferably, the sourceground amplifier is used in consideration of the complexcircuit-structure. However, in view of PSRR (Power Supply RejectionRatio) serving as a ratio for increasing/decreasing an input offsetvoltage depending on the change in power voltage, the source groundamplifier which is operated by the power supply and the ground referencedeteriorates, as compared with the differential amplifier.

Here, a description is given of a specific case of a power layoutpattern of the CMOS image sensor having the amplifier arranged to eachcolumn.

FIG. 14 shows a general layout-structure. Below a pixel area 200, orover and below the pixel area 200, an output circuit area 201 isarranged. In the output circuit area 201, amplifiers AP ( . . . APn,APn+1 . . . APm) are arranged to columns of vertical signal lines VL ( .. . VLn, VLn+1 . . . VLm) and therefore a large number of amplifiers APare arranged in parallel with each other.

To each of the large number of amplifiers AP, a power supply line LVDD,a ground line LGND, or a reference signal line (not shown) of thereference voltage Vref need to be arranged. This wiring arrangement islong in parallel from PADs (power PAD and GND-PAD) on a substrate.

Then, the wiring structure becomes long and therefore the influence of aparasitic resistance r is not ignored.

In the case of using the power supply line LVDD and the ground lineLGND, the current of the amplifier AP flows and therefore the influenceof the parasitic resistance r causes the IR drop (voltage reduction).The potentials of power supply and ground differ between the center andthe end.

Since the current flows to a line having impedance of the parasiticresistance r, the potential is always stable by setting the flowingcurrent value to be constant. However, in the case of the source groundamplifier shown in FIG. 9, the transistor Tr17 has a transmittingconductance as the resistance and sets it as the load. Therefore, thetransistor Tr17 operates like so-called resistance load and the currentvalue flowing depending on the output value of the amplifier changes.

Referring to FIG. 12, a constant-current source is used as a loadtransistor T10. However, the constant-current source is limited in thevoltage range of the operation as the constant-current source. Anexcessively-high-level signal is generated and then the output valuebecomes too higher. In this case, the operating area of the currentsource MOS transistor enters a linear area and the load transistor T10operates like the resistance.

The operating range will be described in the case of using the currentsource as the load with reference to FIGS. 15A and 15B. For a briefdescription, the regulated cascade as shown in FIG. 12 is not used and asimple current-source as the load is used.

Referring to FIG. 15A, a voltage Vbp1 is applied to a gate of theP-channel MOS transistor T10 and an operating voltage is set within asaturated area, thereby functioning as the current source.

The load transistor T10 functions as the current source whenVout<Vbp1+Vtp (where reference symbol Vout denotes the output voltage ofthe amplifier and reference symbol Vtp denotes a threshold voltage ofthe PMOS load transistor T10). FIG. 15B shows a graph using the abscissaaxis as the output voltage Vout and the ordinate axis as current Ids.When the output voltage Vout is over a value of Vbp1+Vtp, the currentIds via the amplifier reduces. Finally, when the output voltage Vout isover a value of the power voltage VDD−Vtp, the current Ids is zero.

As mentioned above, even in the case of using the load transistor T10 asthe current source, the level of the pixel signal from the pixel GS ishigh, a high-level signal is inputted as an input voltage Vin. In thiscase, the output voltage Vout of the amplifier is excessively high andthen the current Ids via the amplifier changes. The input signal is notso high and then the output value is soon saturated when the gain isobtained by the ratio C1/C2. Similarly, the current via the amplifierchanges.

When the current value varies depending on the amount of signals, thelevels of power supply and ground change, thus causing a problem thatthe black level changes and the change appears in the image signal.

In the examples shown in FIGS. 11 and 13, upon reading the offset signalVo at the reset level of the pixel GS, the threshold voltage Vt needs tobe not changed after that. However, the threshold voltage Vt isgenerated based on the power and the ground level as the reference.Therefore, when the signal is too high and the current Ids changes uponreading the signal level Vps, the levels of power supply and groundchange and thus the threshold voltage Vt simultaneously changes.

As a consequence, in order to compensate for the change, the outputvoltage Vout of the amplifier changes and it seems that the black levelchanges.

More seriously, the number of amplifiers AP corresponding to the numberof columns in the horizontal direction shares the impedances of thepower supply and ground, the current value of any amplifier AP changes,and then the change influences on the entire amplifiers AP.

For example, strong light is partly received to the pixel area 200, thecurrent of the amplifier AP for reading the area changes, then, theinfluence is shared in the horizontal direction, and the image signalhaving a horizontal stripe is outputted. Since the part which receivesthe strong light is viewed as white, the slight change in black leveldoes not need to be considered. The part is shared with the peripheraldark portion and then the change in black level becomes the horizontalstripe and this clearly appears in the image. The appearance of stripeis a serious damage for the image sensor and becomes a problem in thecase of using the single-end amplifier.

In order to solve the above-mentioned problems in the case of using thesingle-end amplifier of the source ground amplifier, the levels of powersupply and ground need to be constant and, in any case, the current viathe amplifier needs to be constant.

SUMMARY OF THE INVENTION

In order to solve the above problems, it is an object of the presentinvention to provide a solid-state image pick-up device and a pixelsignal reading method. That is, in an image signal reading circuitsystem having an amplifier arranged to each column, irrespective of thelevel of pixel signal (strength of light) and the gain of amplifier(C1/C2 of a capacitance integrating amplifier in this case), the currentof the amplifier is always constant. Therefore, if the parasiticresistance exists, the levels of power supply and ground are constantand the problems of the change in black level and the horizontal stripeare prevented.

According to one aspect of the present invention, a solid-state imagepick-up device comprises: an image pick-up pixel element having imagepick-up pixels in the row and column directions; a vertical transferelement which outputs, to a vertical signal line arranged to eachcolumn, a pixel signal from the image pick-up pixel in each column ofthe row selected; a plurality of amplifier element each of which isarranged to the vertical signal line of each column and receives thepixel signals from the image pick-up pixels of each column; a limiterelement which limits an output voltage range of the amplifier element,and a plurality of which are arranged to the plurality of amplifierelement; and a horizontal transfer element which transfers, by ahorizontal signal line, the pixel signal outputted by the each amplifierelement.

The limiter element comprises an MOS transistor in which a sourceterminal thereof is connected to an output terminal of the amplifierelement, a drain terminal thereof is connected to the power supply orthe ground, and a gate terminal thereof is connected to a referencepotential for determining a limiter level.

The amplifier element comprises: a current source MOS transistor fordetermining the consumption current; a cascade MOS transistor forsuppressing a drain-terminal voltage of the current source MOStransistor; and an auxiliary amplifier unit which detects a drainvoltage of the current source MOS transistor and applies the feedback toa gate terminal of the cascade MOS transistor so as to keep the drainvoltage constant. Then, the limiter element comprises an MOS transistorin which a source terminal thereof is connected to an output terminal ofthe amplifier element, a drain terminal thereof is connected to thepower supply or the ground, and a gate terminal thereof is connected toa gate terminal of the cascade MOS transistor.

According to another aspect of the present invention, a pixel signalreading method comprises the steps of: outputting, to a vertical signalline arranged to each column, a pixel signal from an image pick-up pixelin each column of a row selected by image pick-up pixel element havingimage pick-up pixels in the row and column directions; outputting thepixel signal appearing on the vertical signal while limiting, within arange, an output voltage of the amplifier element arranged to thevertical signal line of each column; and transferring the pixel signaloutputted from the amplifier element by a horizontal signal line.

According to the present invention, the image pick-up signals in eachcolumn of the selected row are read in parallel therewith for thehorizontal blanking period of the horizontal period, from the imagepick-up pixel element (pixel array) comprising the image pick-up pixelsin the row and column directions. The read image pick-up pixel signalsin each column are outputted to the horizontal signal line for the validperiod of the horizontal period via the amplifier element arranged toeach column (vertical signal line), and are horizontally transferred.

In this case, the amplifier element comprises the limiter element at theoutput thereof to limit the output range of the amplifier element forsetting the consumption current of the amplifier element to be constant.That is, it is possible to prevent the output range of the amplifierelement which reduces the consumption current of the amplifier element.

According to the present invention, in the solid-state image pick-updevice such as the CMOS image sensor, when the signals are read byarranging the amplifier to each column, the limiter element is arrangedto the output terminal of the amplifier element. Thus, the consumptioncurrent of the amplifier element is always kept to constant,irrespective of the level of pixel signal and the gain of the amplifierelement. When the parasitic resistances of power line and ground lineaffect any influence, the potentials of power line and ground line arekept to be constant. Thus, it is possible to solve the problems of thechange in black level of the image signal and in stripe when the stronglight is partly received.

In the case of the single-end amplifier with the simple structure whichis weak in change in levels of the power supply and the ground, thelimiter element is arranged and thus the amplifier is used without anyproblems. Under the limiting condition that the amplifier element islaid-out to each column corresponding, to the pixel pitch, thesingle-end amplifier contributes toe reduction in layout area.

Further, when the amplifier element has the regulated-cascade structure,that is, when the amplifier element comprises a current source MOStransistor for determining the consumption current, a cascade MOStransistor serially connected to suppress a drain terminal voltage ofthe current source MOS transistor, and an auxiliary amplifier unit forfeedback to a gate terminal of the cascade MOS transistor so as todetect a drain voltage of the current source MOS transistor and to keepthe constant drain voltage, the limiter element comprises an MOStransistor having a source terminal connected to the output terminal ofthe amplifier element, a drain terminal connected to the power supplyand the ground, and a gate terminal connected to the cascade MOStransistor. Thus, the certain limiter-operation is guaranteedirrespective of the variation in devices. The margin is increasedagainst a problem of the leakage of limiter circuit in the normaloperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a main portion of a solid-state imagepick-up device according to the present invention;

FIG. 2 is an explanatory diagram of the connecting structure of anamplifier and a limiter according to the present invention;

FIG. 3 is an explanatory diagram of an example of an amplifier/limitercircuit according to the first embodiment;

FIGS. 4A and 4B are explanatory diagrams of the limiter operationaccording to the first embodiment;

FIG. 5 is an explanatory diagram of a limit level and an A/D convertingrange according to the first embodiment;

FIG. 6 is an explanatory diagram of an example of an amplifier/limitercircuit according to the second embodiment;

FIGS. 7A and 7B are explanatory diagrams of the limiter operationaccording to the second embodiment;

FIG. 8 is an explanatory diagram of an example of an amplifier/limitercircuit according to the third embodiment;

FIG. 9 is an explanatory diagram of the structure according to oneconventional art;

FIG. 10 is an explanatory diagram of an operating timing according tothe one conventional art;

FIG. 11 is an explanatory diagram of the structure according to anotherconventional art;

FIG. 12 is an explanatory diagram of an amplifier according to the otherconventional art;

FIG. 13 is an explanatory diagram of an operating timing according tothe other conventional art;

FIG. 14 is an explanatory diagram of the layout of the amplifier and aparasitic resistor; and

FIGS. 15A and 15B are explanatory diagrams of the current characteristicof the amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, a description is given of a solid-state image pick-updevice and a pixel signal reading method in the following order.

1. Entire common structure according to the first to third embodiment

2. Structure of amplifier/limiter according to the first embodiment

3. Structure of amplifier/limiter according to the second embodiment

4. Structure of amplifier/limiter according to the third embodiment

5. Advantages according to the first to third embodiments andmodification

1. Entire Common Structure According to the First to Third Embodiment

FIG. 1 is a block diagram showing a main portion of a solid-state imagepick-up device according to the present invention.

Referring to FIG. 1, light from a subject is incident on a pixel array 1via a lens system (not shown). The pixel array 1 is a CMOS sensor array,and comprises a large number of image pick-up pixels GS serving assolid-state image pick-up devices (CMOS sensors) in the row and columndirections.

A vertical scanning circuit 3 selects and scans the row in the pixelarray 1 based on an address and a control signal supplied from a timinggenerator 2. According to the present invention, the rows aresequentially scanned for selection by reading the pixel in a valid areain the column parallel form. For the operation, the vertical scanningcircuit 3 drives vertical scanning lines L1 to Ln.

The structure of pixel GS may be shown in FIG. 9 or 11. Referring toFIG. 9, the pixel GS comprises: a photodiode PD; a reset transistorTrst; an amplifying transistor Tg; and a reading transistor Ts.Alternatively, referring to FIG. 11, the pixel GS comprises: aphotodiode PD; a reset transistor Trst; an amplifying transistor Tg; areading transistor Ts; and a transfer gate Tt.

As shown in FIG. 1, one vertical scanning line L corresponds to thepixels in one row. In the structure of pixel GS shown in FIG. 9, onevertical scanning line L include a signal line for supplying pulses φRand φSV. In the pixel GS shown in FIG. 11, one vertical scanning line Lincludes a signal line for supplying pulses φR, φSV, and φTX.

According to the present invention, in order to read the pixels in thecolumn parallel form, signal charges from pixels G arranged in the rowdirection in the pixel array 1 are simultaneously read and then aresupplied to a vertical signal line VL (VL1, VL2 . . . ).

Specifically, the vertical scanning circuit 3 enables a signal at thereset level to be supplied to the vertical line VL from the pixels GS inthe selected row (that is, P-phase reading), and then executes theoperation for supplying the pixel signal in accordance with the chargesstored in the photodiode PD to the vertical signal line VL (that is,D-phase reading). The vertical scanning circuit 3 sequentially executesthe reading operation.

The pixel signal from the pixel GS in the selected row is read for ahorizontal blanking period within one horizontal period. That is, forthe horizontal blanking period, the pixel signals from the pixel GS inthe row selected by the vertical scanning circuit 3 are outputted inparallel with the vertical signal lines VL1, VL2 . . . .

The pixel signal transferred to the vertical signal line VL from thepixel array 1 is processed by a column reading unit 5.

The column reading unit 5 comprises: an amplifier AP; a limiter LM; anda switch SW at each vertical signal line VL (column). Although thespecific structure of amplifier AP and limiter LM will be describedlater, the amplifier AP comprises a capacitor, a switch element, and acharge integrating amplifier comprising a single-end amplifier. Theamplifier AP forms a so-called CDS (Correlated Double Sampling) circuitfor sampling the pixel signal. Specifically, the differential between aP-phase reading level (reset level) and a D-phase reading level (datalevel) is sampled as the pixel signal and is outputted.

An output voltage of the amplifier AP is limited by a limiter LM.

The pixels signals are read in parallel from the row selected for thehorizontal blanking period within the horizontal period. The pixelsignals in the column sampled by the column reading unit 5 istransferred to a horizontal signal line HL for a horizontal transferperiod within the horizontal period by sequentially selecting switchesSW by the horizontal scanning circuit 6 and are supplied to the outputcircuit 4.

The horizontal signal line HL includes, e.g., three horizontal signallines HL1, HL2, and HL3. The amplifiers AP (and the limiters LM) in thecolumns are connected via switches SW to be distributed to the threehorizontal signal lines HL1, HL2, and HL3 in the order. In thehorizontal transfer, the amplifier AP drives the horizontal signal lineHL. The amplifiers AP arranged to each column sequentially drive thehorizontal signal lines HL1, HL2, and HL3. A multiplexer MPXsequentially selects and transfers the signals (output of the amplifierAP) to the horizontal signal lines HL1, HL2, and HL3. The signals becomean image pick-up signal in one line and are supplied to the outputcircuit 4.

A plurality of horizontal signal lines HL are arranged because theamplifier AP arranged to each column does not drive one horizontalsignal line fast. That is, a plurality of horizontal signal lines HL arearranged and the signals are horizontally transferred in parallel. Thedata rate per one horizontal signal line is reduced. Thus, the amplifierAP may have a relatively low driving capacity and, advantageously, thelayout area is reduced.

The number of horizontal signal lines is three as one example, and maybe two or four or more. Obviously, the data rate per horizontal signalline is reduced as long as the number of horizontal signal lines islarger. Generally, it is efficient and preferable that the number ofhorizontal signal lines is provided to have the same driving speed asthat of the reading operation for the horizontal blanking period.

When the driving capacity of the amplifier AP is high, the signals maybe horizontally transferred by one horizontal signal line.

The output circuit 4 performs, e.g., AGC processing and clampprocessing, thereby obtaining an image pick-up signal for one horizontalperiod as a serial signal. Further, the output circuit 4 performs A/Dconversion or the like, thereby obtaining an image pick-up signal asdigital data. Furthermore, the digital signal processing is executed,including the digital gain processing and the white balance processing.

Through the processing, the image pick-up signal outputted from theoutput circuit 4 is subjected to the signal processing displayoperation. A display unit displays the image. Further, through theformatting processing and encoding processing, the data is recorded to arecording medium, is sent, and is outputted.

The timing generator 2 controls the operating timings of the verticalscanning circuit 3, the horizontal scanning circuit 6, the columnreading unit 5, and the output circuit 4. The timing generator 2controls the operating timings of the circuits and the units based on avertical sync signal and a horizontal sync signal.

In the solid-state image pick-up device with the above-mentionedstructure, the column reading unit 5 having the amplifier AP arranged toeach column comprises the limiter LM and thus the current of theamplifier AP is always kept to be constant, irrespective of the level ofpixel signal (strength of light) and the gain (C1/C2) of the amplifierAP in this case. As a consequence, the parasitic resistance exists andthen the constant levels of power supply and ground prevent the problemsof change in black level and horizontal stripe.

FIG. 2 shows the structure of the amplifier AP and limiter LM arrangedto each column.

The column reading unit 5 comprises one amplifier AP ( . . . APn, APn+1. . . APm) to each column of the pixel array 1, that is, to the verticalsignal line VL ( . . . VLn, VLn+1 . . . VLm), and further comprises onelimiter LM ( . . . LMn, LMn+1 . . . LMm) thereto.

A power supply line LVDD, a ground line LGND, and another wiring areformed to the above-arranged many amplifiers AP. The amplifiers AP arelong wired from PADs (power PAD and GND-PAD) on a substrate in thehorizontal direction. In this case, as mentioned above, the long wiringaffects the influence of the parasitic resistance r and changes theconsumption current of the amplifier AP, thereby causing the problem.

In the example, the amplifiers AP ( . . . APn, APn+1 . . . APm) arrangedto the columns read the signals outputted to the vertical signal linesVL ( . . . VLn, VLn+1 . . . VLm) in the columns from the pixels GS. Thevoltage of the read signal is limited by the limiter LM ( . . . LMn,LMn+1 . . . LMm) arranged to the back so as to prevent a state in whichit is not over one reference level.

The reference level is within the range for continuously flowingconstant current, irrespective of the output level of the amplifier AP.Preferably, the reference level is out of the input range of the A/Dconverter arranged to the latter stage so as to prevent the influence tothe image pick-up signal.

In the example shown in FIG. 2, the limiter LM is arranged independentlyat the back of the amplifier AP. However, the boarder between thelimiter LM and the amplifier AP may not be clear. That is, theamplifiers AP and the limiters LM mixedly exist and thus the amplifierAP may not output the signal at one level or more.

Hereinbelow, a description is given of the specific structure of theamplifier AP and the limiter LM.

2. Structure of Amplifier/Limiter According to the First Embodiment

The structure of the amplifier AP and the limiter LM will be describedaccording to the first embodiment with reference to FIGS. 3 to 5.

FIG. 3 shows a circuit example of the amplifier AP and the limiter LM inone column of the pixel array 1.

Basically, the amplifier AP is a resource ground amplifier comprising anN-channel MOS driving transistor T2 and a P-channel MOS current sourceload transistor T1, and capacitors C1 and C2.

Incidentally, FIG. 3 shows the example without any regulated cascade ofthe resource ground amplifier. The current source load transistor T1functions as a current source of the current Ids by receiving a gatevoltage Vbp1 and setting an operating voltage within a saturated are.

The capacitors C1 and C2 form the charge integrating amplifier. Thecapacitor C2 is a variable capacitor comprising capacitors c21 and c22and a switch Sφ3, thereby changing the reading gain.

The switch Sφ1 is a reset switch which initializes the capacitor C2.

The limiter LM comprises only the P-channel MOS transistor T3. A voltageVLIMIT for setting a reference level using the limiter is applied to agate of the transistor T3. A source of the transistor T3 is connected anoutput terminal Vout of the amplifier AP, and a drain of the transistorT3 is connected to the ground.

The transistor T3 is conductive when the output voltage Vout is higherthan a predetermined limiter level VLL (=VLIMIT+Vtpl), and then thecurrent Ids via the transistor T1 is shut off. The voltage Vtp1 is athreshold voltage of the transistor T3.

Hereinbelow, a description is given of the operation of the amplifier APand the limiter LM with reference to FIGS. 4A and 4B.

The amplifier AP inputs an input voltage Vps of the signal from thepixel GS, and sets the signal Vps to an output voltage Vout by the gainserving as a capacitance ratio (C1/C2).

If the limiter LM (transistor T3) is not arranged, the amplifier shownin FIG. 3 is the same as that shown in FIGS. 15A and 15B according tothe conventional arts. When the output voltage Vout is higher than avoltage (Vbp1+Vtp), the transistor T1 enters a linear area and thus doesnot function as the current source. Reference symbol Vbp1 denotes a gatevoltage of the transistor T1, and reference symbol Vtp denotes athreshold voltage of the transistor T1.

Referring to FIGS. 4A and 4B, the structure without the limiter LM isshown by a broken line.

Referring to FIG. 4A, the abscissa axis is the input voltage (signalVps) of the amplifier AP, and the ordinate axis is the output voltageVout. In this case, until the output voltage Vout reaches a voltage(Vbp1+Vtp), the input/output characteristic is indicated by theinclination of the gain serving as the capacitance ratio (C1/C2).However, the output voltage Vout is higher than the voltage (Vbp1+Vtp),the input/output characteristic is shown by a curve as shown in FIG. 4A.Referring to FIG. 4B, the abscissa axis is the output voltage Vout, andthe ordinate axis is the current Ids. The output voltage Vout is higherthan the voltage (Vbp1+Vtp) and then the current Ids of the amplifier APdecreases.

On the contrary, according to the first embodiment, the limiter LM isarranged, when the output voltage Vout is higher than a limiter levelVLL (=VLIMIT+Vtpl), the transistor T3 is conductive. The current Ids viathe transistor T1 is shut off, thereby preventing the increase in outputvoltage Vout.

As shown by a bold line in FIG. 4A, the function of the limiter LMprevents a state in which the output voltage Vout is not higher than thevoltage (VLIMIT+Vtpl) as the top level.

As will be understood with reference to FIG. 4B, the limiter LM limitsthe output voltage Vout shown by the bold line to the voltage(VLIMIT+Vtpl) serving as the top level and thus the output voltage Voutvaries within the range for keeping the constant current Ids.

In other words, the voltage VLL (=VLIMIT+Vtpl) as the limiter level isset to be lower than the voltage (Vbp1+Vtp), thereby guaranteeing thatthe transistor T1 is always within the saturated area. It is possible tokeep the constant consumption current for the entire structure of theamplifier AP and the limiter LM.

With the above-mentioned structure, as shown in FIG. 2, the limiter LM (. . . LMn, LMn+1 . . . LMm) limits the output of the amplifier AP ( . .. APn, APn+1 . . . APm) so that it is not over a reference level. Thus,the amplifier AP continuously flows the constant current Ids,irrespective of the output level. In this case, in order to prevent theinfluence on the image pick-up signal from the limiter operation,preferably, the top limit of the output voltage Vout, namely, thelimiter level VLL is set out of the input range of the A/D converterarranged at the latter stage (e.g., in the output circuit 4 FIG. 1).

FIG. 5 shows the A/D converting range of limiter level according to thefirst embodiment. In this case, the limiter LM limits the top voltage ofthe amplifier AP. Normally, the input range of the A/D converter uses alinear input/output characteristic of the amplifier AP. Within the inputrange of the A/D converter, reference symbol RT denotes a top limit andreference symbol RB denotes a bottom limit. Then, the limiter level VLLis higher than the top limit RT and does not change the current value ofthe amplifier AP, namely, is lower than the voltage Vbp1+Vtp.

By the setting, the output voltage Vout is clipped within the range ofvoltage higher than the limiter level VLL, thereby preventing theinfluence on the image. Because the output voltage Vout clipped by thelimiter LM is higher than the limiter level VLL without the limiter LMand the output voltage Vout is clipped over the top limit of the rangeof the A/D converter. In other words, the limiter level VLL is set outof the input range of the A/D converter, thereby limiting, by thelimiter LM, the output voltage Vout serving as the voltage clipped bythe A/D converter. Thus, the presence/absence of the limiter LM does notinfluence on the output of the A/D converter. The limiter operation doesnot influence on the image pick-up signal.

If the limiter level VLL is not freely set, the top limit RT and bottomlimit RB of the A/D converter may change. Generally, the top limit RT isset to the same level as the saturated level of the pixel signal by thelowest gain of the amplifier AP determined depending on the ratio C1/C2,and the bottom limit RB is set to the same level at the dark part.

Although the arrangement position of the A/D converter is not limited,as shown in FIG. 5, the amplifier AP is DC-connected to the A/Dconverter. The arrangement of the gain component or the DC offsetbetween the amplifier AP and the A/D converter needs the setting of thelimiter level.

In the structure shown in FIG. 3, strictly, after the transistor T3 isconductive, the flow of the entire current Ids of the transistor T1 doesnot stop soon. Therefore, the output voltage Vout slightly andcontinuously increases. However, the increase in output voltage Voutresults in the rise in voltage Vgs between the gate and the source ofthe transistor T3 and therefore, in the future, the transistor T3completely stops the flow of the current Ids of the transistor T1 andthe increase in output voltage Vout entirely stops.

The “small increase in output voltage Vout” is determined depending onthe transmitting conductance of the transistor T3, and must properly bedetermined depending on the margin between the voltages VLIMIT+Vtpl andVbp1+Vtp.

If the gate voltage VLIMIT of the transistor T3 is excessively low sothat the limiter is operated without fail, the output range of theamplifier is narrow and may interfere with the top limit RT of the A/Dconverter. Further, if the size of the transistor T3 is excessivelylarge so as to reduce the “small increase in output voltage Vout”, theleakage of current during the normal operation might rise.

3. Structure of Amplifier/Limiter According to the Second Embodiment

FIG. 6 shows the structure of the amplifier AP and the limiter LMaccording to the second embodiment.

Unlike the structure shown in FIG. 3, the amplifier AP uses theregulated cascade. That is, in addition to the structure shown in FIG.3, the amplifier AP comprises a P-channel MOS transistor T4cascade-connected to a current source load transistor T1. Further, theamplifier AP comprises an auxiliary amplifier AP comprising a P-channelMOS transistor T5 and a current source I1.

The above-structured amplifier AP uses a limiter LM comprising only theP-channel MOS transistor T3 shown in FIG. 3. However, the amplifier APshown in FIG. 6 does not use the reference gate voltage VLIMIT. In thelimiter LM shown in FIG. 6, a source terminal of a P-channel MOStransistor T6 is connected to the output terminal of the amplifier AP, adrain terminal thereof is connected the ground, and a gate terminalthereof is connected to a gate terminal (output terminal of theauxiliary amplifier As) of the cascade MOS transistor T4.

Referring to FIG. 6, in the regulated cascade, a drain source voltageVds(T3) of the transistor T3 serving as the current source is a voltageVgs(T5) between the drain source and the gate source of the transistorT5 of the auxiliary amplifier As.

The auxiliary amplifier As applies the negative feedback to thetransistor T4 so that the voltage Vgs(T5) between the gate and thesource is constant. Consequently, a drain source voltage Vds(T1) of thetransistor T1 is constant. If the drain source voltage Vds(T1) of thetransistor T1 is a voltage (i.e., voltage lower than the voltage(Vbp1+Vtp)) for operating the transistor T1 within a saturated area, thetransistor T1 flows the constant current against the change in outputvoltage Vout.

However, the negative feedback is applied to the transistor T4 only whenthe transistor T4 is within the saturated area. The output voltage Voutincreases and then a drain source voltage Vds(T4) of the transistor T4decreases and enters the linear area. Thus, the transmitting conductanceof the transistor T4 reduces and the feedback gain of the auxiliaryamplifier As also drops. As a consequence, the change in output voltageVout is transmitted as the change in drain source voltage Vds(T1) of thetransistor T1.

The auxiliary amplifier As is the source ground amplifier comprising oneP-channel MOS transistor T5 and, normally, has the gain of,approximately, 50 times. Then, the transistor T4 enters the linear areaand then the drain source voltage Vds(T1) of the transistor T1 changes.In this case, the output of the auxiliary amplifier As changes by 50times of the voltage Vds(T1). Hence, the output of the auxiliaryamplifier As has a function of a flag indicating a sign before thetransistor T1 enter the linear area.

For advantageously using the function, in the limiter LM, the gate ofthe P-channel MOS transistor T6 is connected to the output terminal ofthe auxiliary amplifier (namely, the drain of the transistor T5), thesource thereof is connected to the output voltage Vout, and the drainthereof is connected to the ground.

The transistor T6 of the limiter LM enables the change in drain sourcevoltage Vds(T1) of the transistor T1 to be multiplied by the time of thegain of the auxiliary amplifier As, thereby changing a voltage Vgs(T6)between the gate and the source of the transistor T6. The transistor T6is conductive before the transistor T1 enters the linear area. Thecurrent Ids does not flow and thus the transistor T1 exists within thesaturated area.

According to the first embodiment, the limiter level is controlled byapplying the reference voltage VLIMIT to the gate of the transistor T3in the limiter LM. In this case, the voltage VLIMIT must be determinedin consideration of the threshold voltage Vth of the transistor T3, thethreshold voltage Vth of the transistor T1, and “the small increase ofoutput voltage Vout” determined depending on the transmittingconductance of the transistor T3.

On the contrary, according to the second embodiment, as shown in FIG. 6,the transistor T1 detects by itself that it enters the linear area andsets by itself the limiter. The limiter is certainly set, irrespectiveof the variation in devices.

Further, according to the first embodiment, the gate of the transistorT3 has a fixed potential serving as the voltage VLIMIT. On the otherhand, according to the second embodiment, the output voltage of theauxiliary amplifier As is active. In this case, the amount of change inthe voltage Vgs(T6) between the gate and the source is large. Even forthe smaller size of transistor, the current Ids of the transistor T1stops. This is advantageous against the leakage of current in the normaloperation.

FIGS. 7A and 7B show operating examples according to the secondembodiment. Incidentally, the abscissa and the ordinate in FIGS. 7A and7B are similar to those in FIGS. 4A and 4B. Similarly, upon using thelimiter LM, the input/output characteristic and the outputvoltage-current characteristic are shown by bold lines.

According to the second embodiment, the structure of the amplifier APshown in FIGS. 7A and 7B is different from that shown in FIG. 6according to the first embodiment. However, the amplifier AP uses thecharge integrating amplifier comprising the capacitors C1 and C2 both inFIG. 6 and FIGS. 7A and 7B according to the first and secondembodiments. The input signal Vps is read by using the gain C1/C2.

The voltage VLL for using the limiter starts to change the drain sourcevoltage Vds(T1) of the transistor T1. Thus, the voltage VLL enables thetransistor T4 to enter the linear area.

The source voltage of the transistor T4 is determined depending on thevoltage Vgs(T5) between the gate and the source of the transistor T5,and is a power voltage VDD−Vgs(T5). The output voltage Vout for enablingthe transistor T4 to enter the linear area is a voltage[VDD−Vgs(T5)−Vgs(T4)+Vth(T4)]. Incidentally, this relationship isestablished when threshold voltage Vth (T4) of the transistorT4<threshold voltage Vth(T6) of the transistor T6.

On the contrary, when Vth(T4)>Vth(T6), the transistor T6 is conductivebefore the transistor T4 enters the linear area and the operation is thesame as that according to the first embodiment. In this case, the gatevoltage of the transistor T6, corresponding to the reference voltageVLIMIT, according to the first embodiment is a voltage[VDD−Vgs(T5)−Vgs(T4)]. Therefore, the limiter operates when the voltageis [VDD−Vgs(T5)−Vgs(T4)+Vth(T6)].

In this case, the limiter acts before the transistor T4 enters thelinear area. Therefore, the limiter operates slightly fast. Therefore,in the case of Vth(T4)<Vth(T6), the output range of the amplifier AP isadvantageously used, an effective voltage [Vgs(T6)−Vth(T6)] of thetransistor T6 in the normal operation is small. This is advantageousagainst the leakage of voltage.

In the case of using the same P-channel MOS transistor, the largerback-bias is applied to the transistor T6, as compared with thetransistor T4 and therefore a relationship of Vth(T4)<Vth(T6) isobtained without any operation. Preferably, the relationship ofVth(T4)<Vth(T6) is established by adjusting the transistor size or theconcentration of impurity in consideration of the variation.

4. Structure of Amplifier/Limiter According to the Third Embodiment

FIG. 8 shows the structure of an amplifier AP and a limiter LM accordingto the third embodiment.

The structure shown in FIG. 8 is different from that according to thesecond embodiment in two regulated cascades.

Referring to FIG. 8, the P-channel MOS transistor T4 cascade-connectedto the current source load transistor T1 is arranged, and the amplifierAP comprises an auxiliary amplifier As1 comprising a P-channel MOStransistor T5 and a current source I1.

Further, an N-channel MOS transistor T7 cascade-connected to the drivingtransistor T2 is arranged. The amplifier AP comprises an auxiliaryamplifier As2 comprising an N-channel MOS transistor T8 and a currentsource I2.

The structure of the limiter LM shown in FIG. 8 is the same as thatshown in FIG. 6, and the operation of the limiter LM is the same as thatshown in FIGS. 7A and 7B.

As shown in FIG. 8, the addition of the regulated cascade to the drivingtransistor T2 enables the use of the limiter LM without any trouble. Theamplifier AP with the above-mentioned structure can have the limiter LMshown in FIG. 3 according to the first embodiment.

5. Advantages of Embodiments and Modifications

According to the first to third embodiments, when the high input-voltageVps is applied from the pixel GS so that the current Ids via theamplifier AP changes, the limiter LM limits the output voltage Voutwithin the range of no-change in current Ids, and thus the levels of thepower and the ground are constant. Therefore, the black level is alwaysconstant and the problem of the stripe is solved when the strong lightis partly received to the screen.

Further, the single-end amplifier weak in change in power supply andground can be used without any trouble. Under the limiting condition ofthe layout corresponding to the pixel pitch for each column, thesingle-end amplifier with the simple structure contributes to thereduction in layout area.

Although the amplifier AP may be a differential amplifier,advantageously, the single-end amplifier is used according to thepresent invention.

The amplifier AP having the regulated cascade uses the limiter LMaccording to the second and third embodiments, thereby guaranteeing thelimiter operation without fail, irrespective of the variation. Themargin is increased against the problem of the leakage of the limitercircuit in the normal operation.

According to the first to third embodiments, the amplifier comprises theN-channel MOS transistor serving as the driving transistor and theP-channel MOS transistor serving as the current source transistor, andthe limiter circuit limits the top level of amplifier output voltageVout.

Of course, the above-mentioned structure depends on the structure of theamplifier AP. Therefore, the amplifier AP may comprise the P-channel MOStransistor serving as the driving transistor and the N-channel MOStransistor serving as the current source transistor and then the limitercircuit limits the bottom level of amplifier output Vout. In this case,the limiter LM comprises the N-channel MOS transistor.

Further, depending on the structure of the amplifier AP, the limitercircuit may limit both the top and bottom levels. In this case, both theN-channel MOS transistor and the P-channel MOS transistor function asthe limiters LM.

1-4. (canceled)
 5. A pixel signal reading method comprising: outputting,to a corresponding vertical signal line of plurality of vertical signallines respectively disposed for each column of pixels, a pixel signalfrom an image pick-up pixel of a selected row of pixels, the columns androws of pixels constituting an array of pixels; amplifying the pixelsignal appearing on each vertical signal line and limiting an output ofeach amplifier element for the vertical signal line of each column; andthereafter separately transferring the pixel signals output from theplurality of amplifiers, wherein the step of limiting imposes a boundarylimit on the operating range of each amplifier.
 6. The pixel signalreading method of claim 5, wherein said limiting step includes limitinga consumption current of said amplifier element to be substantiallyconstant.
 7. The pixel signal reading method of claim 5, wherein saidlimiting step includes keeping a current source load transistorcontained in said amplifier element operating in a saturated state. 8.The pixel signal reading method of claim 5, wherein the amplifierincludes a correlated double sampling circuit, and said amplifying stepincludes a step of sampling the differential between a reset readinglevel and a data reading level.
 9. The pixel signal reading method ofclaim 5, further providing a resource ground amplifier as the amplifier.10. The pixel signal reading method of claim 5, further providing acharge integrating amplifier as the amplifier.
 11. The pixel signalreading method of claim 5, further providing a limiter comprising an MOStransistor having a gate terminal connected to a reference potential,the reference potential at least partially determining what voltagelevel to prevent the output of the amplifier from going above.
 12. Thepixel signal reading method of claim 11, wherein the output of theamplifier is limited by the limiter to a voltage equal to VLIMIT+Vthl,wherein VLIMIT is the signal level applied to the gate terminal of theMOS transistor of the limiter, and Vthl is the threshold voltage levelof the MOS transistor of the limiter.
 13. The pixel signal readingmethod of claim 5, wherein the array of pixels comprise a CMOS imager,and the vertical signal lines connecting each image pick-up pixel in acolumn to a respective amplifier is provided as a metal wiring.